It is known to detect a damage of a wind turbine blade on the basis of strain that occurs in a wind turbine blade.
For instance, Patent Document 1 discloses detecting presence or absence of a damage of a wind turbine blade on the basis of wavelength of reflection light from a grating portion of a fiber-optic sensor mounted to the wind turbine blade, the wave length being obtained as data related to strain of the wind turbine blade. Furthermore, Patent Document 1 discloses detecting a damage of a wind turbine blade by using a wavelength fluctuation index indicating a fluctuation amount of the above described wavelength in order to eliminate measurement errors of the fiber-optic sensor accompanying a temperature change in the ambient temperature or the like.